Boundary acoustic wave device manufacturing method

ABSTRACT

A method for manufacturing a boundary acoustic wave device includes the steps of preparing a laminated structure in which an IDT electrode is disposed at an interface between first and second solid media and reforming the first medium and/or the second medium by externally providing the laminated structure with energy capable of reaching the inside of the first medium and/or the second medium and thus adjusting a frequency of the boundary acoustic wave device. The above provides a boundary acoustic wave device manufacturing method that enables frequency adjustment to be readily performed with high accuracy.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for manufacturing a boundaryacoustic wave device utilizing a boundary acoustic wave propagatingthrough an interface between first and second solid media and relates toa boundary acoustic wave device. More specifically, the presentinvention relates to a boundary acoustic wave device manufacturingmethod including a step of adjusting frequency characteristics andrelates to a boundary acoustic wave device.

2. Description of the Related Art

A surface acoustic wave device is widely used as a band-pass filter or aresonator, for example, in a cellular phone. In a surface acoustic wavedevice, an interdigital transducer (IDT) electrode is disposed on apiezoelectric substrate, and the IDT electrode receives and excites asurface acoustic wave. Accordingly, in order to avoid interference withreception and excitation of a surface acoustic wave, it is necessary tohave a space above a portion where the IDT electrode is disposed.

When a surface acoustic wave device is used as a band-pass filter or aresonator, the frequency of the surface acoustic wave device must becontrolled with high accuracy. Accordingly, after the surface acousticwave device is produced, a mass load member may be provided to thepiezoelectric substrate or the electrode may be processed for thepurpose of adjusting the frequency.

Japanese Unexamined Patent Application Publication No. 2001-77661discloses a method by which a frequency can be adjusted after a surfacewhere an IDT electrode of a surface acoustic wave device is disposed issealed with a cap. FIG. 12 is a schematic front cross-sectional view fordescribing the frequency adjusting method described in JapaneseUnexamined Patent Application Publication No. 2001-77661. It is notedthat hatching indicating a cross section is omitted in this drawing,which schematically illustrates the cross section.

According to the frequency adjusting method described in JapaneseUnexamined Patent Application Publication No. 2001-77661, a surfaceacoustic wave device 501 includes a piezoelectric substrate 502, an IDTelectrode 503 disposed on the piezoelectric substrate 502, and a capmember 504 attached above the piezoelectric substrate 502 such that aspace S is present above the IDT electrode 503. That is, an adhesive 505is applied to seal the space S and surround a portion where the IDTelectrode 503 is disposed, and the cap member 504 is bonded to thepiezoelectric substrate 501 by the adhesive 505.

Accordingly, it is difficult to adjust the frequency by processing theportion where the IDT electrode 503 is disposed after the surfaceacoustic wave device 501 is produced. However, in Japanese UnexaminedPatent Application Publication No. 2001-77661, the frequency is adjustedafter the cap member 504 is bonded to the piezoelectric substrate 501.Here, a substance 506 that is vaporized by being heated by a laser beamis applied on the inner surface of the cap member 504 in advance. Thecap member 504 is made of a light-transmitting material that allows alaser beam to pass therethrough. When the surface acoustic wave device501 is irradiated with a laser beam emitted from thereabove using alaser apparatus 507, the laser beam transmits through the cap member504. Accordingly, the substance 506 is heated by the laser beam,vaporized, and deposited on the IDT electrode 503 positioned therebelow.By controlling the irradiation energy of this laser beam, it is possibleto adjust the frequency, according to Japanese Unexamined PatentApplication Publication No. 2001-77661.

Nowadays, attention is being given to a boundary acoustic wave device,instead of a surface acoustic wave device. For example, WO 2004/070946discloses a boundary acoustic wave device that includes an IDT electrodearranged at an interface between first and second solid media and thatuses a boundary acoustic wave propagating through the interface. In thisboundary acoustic wave device, the IDT electrode is arranged at theinterface between the first and second solid media, and there is no needto have a space to which the IDT electrode is exposed.

It is unnecessary for a boundary acoustic wave device to have a space towhich an IDT electrode is exposed. Accordingly, a boundary acoustic wavedevice can be more compact when compared with a surface acoustic wavedevice and can have a simplified package structure.

When a boundary acoustic wave device is used as a filter or a resonator,it is also necessary to set its frequency with high accuracy. However,in the boundary acoustic wave device, an IDT electrode is embedded at aninterface between first and second solid media and there is no space towhich the IDT electrode is exposed. Accordingly, the frequency adjustingmethod for use in the surface acoustic wave filtering device describedin Japanese Unexamined Patent Application Publication No. 2001-77661 isinapplicable to the boundary acoustic wave device.

That is, in the surface acoustic wave device, there is no second mediumabove the IDT, and the surface portion of the IDT and the piezoelectricsubstrate, which allow an elastic wave to pass therethrough, faces aspace. Reforming to change the acoustic velocity of the surface portionof the IDT and the piezoelectric substrate can be performed from thespace side. If such frequency adjustment is applied to a boundaryacoustic wave device, a portion of the second medium that is opposite tothe interface to the first medium would be reformed, and the secondmedium positioned at the interface through which a boundary acousticwave propagates could not be reformed.

Accordingly, traditionally, it has been necessary to manufacture aboundary acoustic wave device that can exhibit desired characteristicswith high accuracy in manufacturing thereof. There is no known effectivemethod for adjusting frequency characteristics after manufacturing.

SUMMARY OF THE INVENTION

In light of the circumstances described above, preferred embodiments ofthe present invention provide a boundary acoustic wave devicemanufacturing method and a resulting boundary acoustic wave device thatenable frequency characteristics to be adjusted with high accuracy.

According to a preferred embodiment of the present invention, a methodfor manufacturing a boundary acoustic wave device includes a firstmedium, a second medium laminated on the first medium, and aninterdigital transducer (IDT) electrode arranged at an interface betweenthe first medium and the second medium and utilizing a boundary acousticwave that propagates through the interface includes the steps ofpreparing a laminated structure in which the IDT electrode is arrangedat the interface between the first medium and the second medium andreforming the first medium and/or the second medium by externallyproviding the laminated structure with energy capable of reaching theinside of the first medium and/or the second medium and thus adjusting afrequency of the boundary acoustic wave device.

In a method for manufacturing a boundary acoustic wave device accordingto a preferred embodiment of the present invention, in the step ofreforming the first medium and/or the second medium by externallyproviding the first medium and/or the second medium with the energy andthus adjusting the frequency of the boundary acoustic wave device, theenergy may preferably be concentrated on a portion of the first mediumand/or the second medium, thus reforming the portion of the first mediumand/or the second medium. In this case, because the concentration of theenergy on the portion of the first medium and/or the second mediumreforms that portion, the frequency can be adjusted more largely andreliably.

A method for manufacturing a boundary acoustic wave device according toa preferred embodiment of the present invention may preferably furtherinclude the step of forming a reform medium layer in the first mediumand/or the second medium. In the step of reforming the first mediumand/or the second medium and thus adjusting the frequency, the reformingmay preferably be performed by the provision of the energy to the reformmedium layer. In this case, forming the reform medium layer from amaterial that is easy to be reformed enables the frequency adjustment tobe performed by the provision of energy more reliably and readily.

In a method for manufacturing a boundary acoustic wave device accordingto a preferred embodiment of the present invention, when a wavelength ofthe IDT electrode is λ, in reforming the first medium and/or the secondmedium and thus adjusting the frequency, the reforming may preferably beperformed by the provision of the energy to a region within a distanceof λ from the interface in a direction in which the first medium and thesecond medium are laminated. Because the energy of a boundary acousticwave is concentrated on the region within a distance of λ from theinterface in the direction in which the first medium and second mediumare laminated, the reforming can be effectively performed by theprovision of the energy to the region within a distance of λ from theinterface.

In a method for manufacturing a boundary acoustic wave device accordingto a preferred embodiment of the present invention, a femtosecond laserbeam may preferably be used as the energy. In such a case, thefemtosecond laser beam can be emitted from outside the first mediumand/or the second medium, and the energy of femtosecond laser beam canbe readily and reliably guided to a region where reforming is desired.

In a method for manufacturing a boundary acoustic wave device accordingto a preferred embodiment of the present invention, a light absorptionwavelength of the reform medium layer may preferably be different from alight absorption wavelength of the first medium and/or the secondmedium, in which the reform medium layer is disposed. The reforming maypreferably be performed by concentration of a laser beam defining theenergy on the reform medium layer, and the laser beam may preferablyhave a wavelength at which the reform medium layer absorbs the laserbeam. In this case, the use of the laser beam having a wavelength atwhich the reform medium layer absorbs the laser beam can prevent theenergy of the laser beam from being absorbed in the first medium and/orthe second medium and enables the energy of the laser beam to beeffectively absorbed in the reform medium layer in reforming.

Preferably, each of the reform medium layer and the first medium and/orthe second medium may be made of a material that is reformed by heat,and a temperature at which the reform medium layer reforms may be lowerthan a temperature at which the first medium and/or the second medium,in which the reform medium layer is disposed, reforms. In this case, thereform medium layer is heated by the energy of laser beam irradiation,for example. A temperature at which the reform medium layer reforms islower than a temperature at which the first medium and/or the secondmedium reforms. Thus, the reform medium layer can be reformed moreeffectively, and frequency characteristics can be adjusted.

A boundary acoustic wave device according to a preferred embodiment ofthe present invention includes a first medium, a second medium laminatedon the first medium, and an IDT electrode arranged at an interfacebetween the first medium and the second medium. The boundary acousticwave device further includes a reformed portion disposed in the firstmedium and/or the second medium, reformed by externally provided energy,and achieving frequency characteristics different from frequencycharacteristics exhibiting when the boundary acoustic wave deviceincludes only the first medium and/or the second medium.

In a boundary acoustic wave device according to a preferred embodimentof the present invention, the reformed portion may preferably beobtained by reforming of a portion of the first medium and/or the secondmedium. In this case, another material is not necessary in reforming theportion of the first medium and/or the second medium. Accordingly,without causing an increase in cost and complication of a manufacturingprocess, a boundary acoustic wave device that has frequencycharacteristics controlled with high accuracy can be provided.

In a boundary acoustic wave device according to a preferred embodimentof the present invention, the reformed portion may preferably beobtained using a reform medium different from the first medium and/orthe second medium and configured by reforming of the reform medium. Inthis case, forming the reformed portion by reforming the reform mediumthat is reformed by the provision of energy more readily than the firstmedium and/or the second medium can adjust frequency characteristicsmore reliably and reliably.

In a boundary acoustic wave device according to a preferred embodimentof the present invention, when a wavelength of the IDT electrode is λ,the reformed portion may preferably be disposed within a distance of λfrom the interface. Because the energy of a boundary acoustic wave isconcentrated on the region within a distance of λ from the interface inthe direction in which the first medium and second medium are laminated,when the reformed portion is disposed in the region within a distance ofλ from the interface, frequency characteristics can be adjusted morereadily and reliably. Accordingly, a boundary acoustic wave device thathas frequency characteristics controlled with higher accuracy can beprovided.

With a method for manufacturing a boundary acoustic wave deviceaccording to a preferred embodiment of the present invention, externallyproviding energy capable of reaching the inside of the first mediumand/or the second medium after preparing the laminated structure inwhich the IDT electrode is disposed between the first and second mediareforms the first medium and/or the second medium, thereby adjusting thefrequency of the boundary acoustic wave device. Accordingly, externallyproviding such energy transmitting through the medium enables thefrequency adjustment of the boundary acoustic wave device to beperformed with high accuracy.

In manufacturing many boundary acoustic wave devices, the adjustment offrequency characteristics of obtained boundary acoustic wave devicesusing the above-described method enables boundary acoustic wave deviceshaving uniform frequency characteristics to be supplied with stability.

A boundary acoustic wave device according to a preferred embodiment ofthe present invention includes the reformed portion disposed in thefirst medium and/or the second medium, reformed by externally providedenergy, and achieving frequency characteristics different from frequencycharacteristics exhibited when the boundary acoustic wave deviceincludes only the first medium and/or the second medium. Accordingly,boundary acoustic wave devices whose variations in frequencycharacteristics are reduced can be obtained simply by frequencyadjustment using externally provided energy after the laminatedstructured is obtained in accordance with a manufacturing method inaccordance with a preferred embodiment of the present invention.

Other features, elements, steps, characteristics and advantages of thepresent invention will become more apparent from the following detaileddescription of preferred embodiments of the present invention withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a method for manufacturing a boundaryacoustic wave device according to one preferred embodiment of thepresent invention.

FIGS. 2A and 2B are a schematic top cross-sectional view and a schematicfront cross-sectional view, that respectively illustrate the boundaryacoustic wave device according to a first preferred embodiment in astate before laser beam irradiation.

FIGS. 3A and 3B are a schematic top cross-sectional view and a schematicfront cross-sectional view, that respectively illustrate the boundaryacoustic wave device obtained according to the first preferredembodiment.

FIG. 4 is a schematic diagram of an apparatus for emitting a laser beamin the first preferred embodiment of the present invention.

FIGS. 5A and 5B are schematic plan views that illustrate a scanningmethod using a laser beam in the first preferred embodiment of thepresent invention.

FIG. 6 is a schematic diagram that schematically illustrates a region onwhich a laser beam is focused according to the first preferredembodiment of the present invention.

FIG. 7 illustrates resonance characteristics of the boundary acousticwave device according to the first preferred embodiment before and afterlaser beam irradiation.

FIG. 8 is a schematic front cross-sectional view describing a modifiedexample of a method for manufacturing a boundary acoustic wave deviceaccording to a preferred embodiment of the present invention.

FIG. 9 is a schematic front cross-sectional view describing anothermodified example of a method for manufacturing a boundary acoustic wavedevice according to a preferred embodiment of the present invention.

FIG. 10 illustrates dependence of transmittance of silicon dioxide onwavelength.

FIG. 11 illustrates dependence of absorptance of silver, copper, andgold on wavelength.

FIG. 12 is a schematic front cross-sectional view that illustrates oneexample of a frequency adjusting method for use in a conventionalsurface acoustic wave device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is described below using explanation of specificpreferred embodiments of the present invention with reference to thedrawings.

FIG. 1 is a front cross-sectional view that schematically illustrates amethod for manufacturing a boundary acoustic wave device according toone preferred embodiment of the present invention.

According to the manufacturing method in the present preferredembodiment, a laminated structure 5 illustrated in FIG. 1 is firstprepared. In the laminated structure 5, a second medium 2 preferablymade of silicon dioxide (SiO₂) is laminated on a first medium 1preferably made of lithium niobate (LiNbO₃). An interdigital transducer(IDT) electrode 3 is arranged between the first medium 1 and the secondmedium 2. The IDT electrode 3 is preferably made of gold.

A reform medium layer 4 is disposed in the second medium 2. When beingsubjected to laser beam irradiation, the reform medium layer 4 isreformed. In the present preferred embodiment, the reform medium layer 4is preferably made of gold. The second medium 2 is preferably made ofSiO₂ and allows light to transmit therethrough.

After the laminated structure 5 illustrated in FIG. 1, i.e., thelaminated structure 5 in which the IDT electrode 3 is arranged at theinterface X between the first medium 1 and the second medium 2 and thereform medium layer 4 is arranged in the second medium 2 is prepared,the laminated structure 5 is irradiated with a laser beam emittedthrough the outer surface of the second medium 2, as indicated by thearrow. The laser beam irradiation heats the reform medium layer 4, andgold forming the reform medium layer 4 is diffused into the secondmedium 2. In such a way, a portion that is reformed by laser beamirradiation, i.e., a reformed portion is produced in the reform mediumlayer 4. The use of the reformed portion enables frequencycharacteristics of a boundary acoustic wave device 6 illustrated in FIG.3 to be adjusted.

Accordingly, after the laminated structure 5 is obtained, the frequencyadjustment can be readily achieved simply by externally applied laserbeam irradiation.

As previously described, in a surface acoustic wave device, there is nosecond medium above an IDT, and the surface region of the IDT and thepiezoelectric substrate through which an elastic wave propagates opposesa space. Reforming that changes the acoustic velocity of the IDT and thepiezoelectric substrate can be achieved from the space side. If suchfrequency adjustment is applied in a boundary acoustic wave device, aportion of the second medium that is opposite to the interface to thefirst medium would be reformed and the second medium positioned at theinterface through which a boundary acoustic wave propagates could not bereformed.

In contrast to this, according to the present preferred embodiment,disposing the reform medium layer 4 in the second medium 2 and makingthe second medium of a light-transmitting material, such as SiO₂, forexample, enables reforming to be performed in the reform medium layer 4by laser beam irradiation. Accordingly, after the laminated structure 5is obtained, the frequency of the boundary acoustic wave device 6 can bereadily adjusted. Thus, boundary acoustic wave devices whose variationsin frequency characteristics are reduced can be provided.

When receiving energy from laser beam irradiation, the reform mediumlayer 4 is reformed in itself. In the above preferred embodiment, thisreforming is achieved by diffusion of gold forming the reform mediumlayer 4 into the surroundings. Accordingly, the above reformingindicates not only changing of the nature of the reform medium layer 4but also reforming of the second medium portion adjacent to the reformmedium layer 4.

In the above preferred embodiment, the reform medium layer 4 is disposedin the second medium 2. However, the reform medium layer 4 may bedisposed in the first medium 1, or alternatively, may be disposed inboth of the first medium 1 and the second medium 2. If the reform mediumlayer 4 is disposed in the first medium, a laser beam can be emittedfrom outside the first medium 1.

The reform medium layer 4 can be made of various kinds of material aslong as the material is reformed by an energy providing unit, such as alaser, for example, more readily than the second medium, in which thereform medium layer 4 is arranged.

A method for manufacturing a boundary acoustic wave device according tothe present preferred embodiment is described more specifically withreference to FIGS. 2A to 8.

FIGS. 2A and 2B are a schematic top cross-sectional view and a schematicfront cross-sectional view that respectively illustrate a detailedstructure of the laminated structure 5 before laser beam irradiation.

The laminated structure 5 has a structure in which the first medium 1and the second medium 2 are laminated. Here, the first medium 1 ispreferably made of 15° Y-cut X-propagation LiNbO₃ as a piezoelectricsingle crystal. When the wavelength determining the pitch of theelectrode fingers of the IDT electrode about 3 is λ, the thickness ofthe first medium 1 is about 4λ to about 100λ.

The second medium 2 is disposed above and below the reform medium layer4 and is preferably made of SiO₂. A lower second medium 2 a disposedbelow to cover the IDT electrode 3 preferably has a thickness of 2μm=0.625λ, and an upper second medium 2 b, which is preferably made ofthe same SiO₂, has a thickness of 4 μm=1.25λ, for example.

The reform medium layer 4 is preferably made of gold and has a thicknessof about 20 nm.

The period, i.e., wavelength λ of the IDT electrode 3 to excite aboundary acoustic wave preferably is about 3.2 μm and the duty is about0.5, for example. As illustrated in FIG. 2A, the IDT electrode 3 iscrossing-width weighted. That is, the cross-width weighting is performedsuch that the crossing-width of the IDT electrode 3 at the centralsection is about 30λ and the crossing-width of the IDT electrode 3 atthe opposite ends in the direction in which boundary waves propagate isabout 12λ, for example. The number of pairs of the electrode fingers ofthe IDT electrode 3 is preferably 50, for example. Reflectors 7 and 8are arranged at the opposite sides of the IDT electrode 3 in thedirection in which boundary waves propagate. The number of the electrodefingers of each of the reflectors 7 and 8 is preferably 51.

The IDT electrode 3 and the reflectors 7 and 8 are formed from alaminated metal film composed predominantly of gold. That is, thelaminated metal film is used in which a NiCr film having a thickness ofabout 10 nm is laminated above and below a gold film having a thicknessof about 150 nm, for example.

In manufacturing, the IDT electrode 3, the reflectors 7 and 8, andwiring patterns 9 and 10 are first formed on the lithium niobate as thefirst medium 1 by photolithography. After that, the silicon dioxidelayer 2 a is preferably formed by sputtering.

Then, a gold film having a thickness of about 20 nm is formed on thesilicon dioxide layer 2 a by electron-beam vapor deposition, forexample, and thus the reform medium layer 4 is formed.

Then, the silicon dioxide layer 2 b is preferably formed on the reformmedium layer 4 by sputtering. In such a way, the second medium 2, inwhich the reform medium layer 4 is disposed and which is made out of thesilicon dioxide layers 2 a and 2 b, is formed.

Then, the silicon dioxide layer 2 b and the reform medium layer 4 onelectrode pads 9 a and 10 a of the wiring patterns 9 and 10 forelectrically connecting to an outside element are removed byphotolithography. In such a way, the electrode pads 9 a and 10 a areexposed, and they can be electrically connected to an outside element.

The electrode pads 9 a and 10 a are connected to a measuring apparatus,and frequency characteristics of the laminated structure obtained insuch a way are measured. After that, the laminated structure 5 isirradiated with a laser beam emitted from above the silicon dioxidelayer 2 b.

An apparatus for emitting a laser beam is illustrated in FIG. 4. Thelaminated structure 5 manufactured in the above-described way before itsfrequency is adjusted is arranged on a stage 11 movable in XYZdirections. The laminated structure 5 is irradiated with a laser beamemitted by a laser apparatus 21 and reflected from a reflecting mirror11. The laser beam being focused by a lens 23 reaches the laminatedstructure 5. In this case, the lens 23 is disposed so as to focus alaser beam such that its energy converges on a portion of the secondmedium 2.

The stage 11 is moved in the XYZ directions while the laminatedstructure 5 is being irradiated with the laser beam, so that the regionirradiated with the laser beam is moved. The region scanned with thelaser beam is a region indicated by alternate long and short dashedlines B schematically illustrated in FIGS. 5A and 5B, i.e., the regionwhere the IDT electrode 3 and the reflectors 7 and 8 are disposed. Asillustrated in FIG. 5B, the region surrounded by the alternate long andshort dashed lines B is scanned with a laser beam at a speed of about 5mm/sec in the direction in which boundary acoustic waves propagate andits opposite direction at intervals of about 1 μm in a directionperpendicular or substantially perpendicular to the direction in whichboundary acoustic waves propagate.

As illustrated in FIG. 6, the energy of a laser beam emitted through thelens 23 is provided such that the focus of the laser beam matches within a reform target portion 2 c in the second medium 2. The reform targetportion 2 c ranges from the height position about 2 μm distant from theinterface X to the height position about 8 μm distant from the interfaceX, for example. The focus matches with the height position about 5 μmdistant from the interface X, for example.

The reason why the focus of the laser beam is positioned about 5 μmabove the interface X, as described above, is that the reform mediumlayer 4 is located inside a reform target region.

A half mirror that allows an irradiation state of a laser beam to beobserved from behind is preferably used as the reflecting mirror 22. Amicroscope 24 is preferably arranged behind the reflecting mirror 22.The laser beam irradiation region is observed by the microscope 24.

An apparatus that can emit a femtosecond laser beam having a centralwavelength of about 1560 nm, a pulse width of about 900 femtoseconds,and a period of about 350 Hz with an output of about 200 mW ispreferably used as the laser apparatus 21, for example. The use of thefemtosecond laser beam enables adjustment of the focus with highaccuracy. Accordingly, the laser beam can be emitted while its energyconverges on a region to be reformed with high accuracy.

In such a way, the energy of a laser beam converges on the region thatmatches the focus, i.e., the reform target region. As a result, asillustrated in a state after laser beam irradiation shown in FIGS. 3Aand 3B, which are schematic top and front cross-sectional views,respectively, in the portion irradiated with the laser beam in thereform medium layer 4, gold is preferably diffused into thesurroundings, and a portion that has a high gold density, i.e., areformed portion 4B expands in the second medium 2 in its thicknessdirection. After such reforming, the electrode pads 9 a and 10 a areconnected to the measuring apparatus, and frequency characteristics ofthe boundary acoustic wave device 6 are measured by the measuringapparatus. The results are illustrated in FIG. 7.

FIG. 7 also illustrates impedance characteristics before the abovereforming step. The resonant resistance, anti-resonant frequency, andanti-resonant resistance before and after laser beam irradiation, andthe amount of change in each of them are listed in Table 1.

TABLE 1 Anti- Anti- Resonant Resonant resonant resonant FrequencyResistance Frequency Resistance (MHz) (dB) (MHz) (dB) After Laser 975.310.79 1031.56 61.0 Beam Irradiation Before Laser 974.38 1.65 1029.6961.61 Beam Irradiation Amount of −0.94 0.86 −1.88 0.60 Change

Accordingly, the results reveal that the resonant frequency and theanti-resonant frequency are reduced by approximately 1 MHz toapproximately 2 MHz by irradiation of the above femtosecond laser beamand a frequency adjustment of approximately 1000 ppm to approximately2000 ppm, for example, can be performed.

Preferably, the material of the reform medium layer 4 may be selectedsuch that the temperature at which the reform medium layer 4 reforms islower than the temperature at which the second medium 2 reforms. In thiscase, when the temperature of the reform medium layer 4 is increased byenergy of a laser beam, the adjustment of the energy of the laser beamenables the reform medium layer 4 to be reformed at a temperature lowerthan that of the second medium 2. Accordingly, the frequency adjustmentcan be achieved more readily and reliably.

Preferably, the light absorption wavelength of the reform medium layer 4may differ from the light absorption wavelength of the second medium 2,and light having a wavelength at which the reform medium layer 4 absorbsthe light may be used as the above laser beam. In this case, during thelaser beam irradiation, its energy is not absorbed in the second medium2, and the reform medium layer 4 absorbs that energy and is reformed.Accordingly, the frequency adjustment can be readily and reliablyachieved simply by selection of the wavelength of the laser beam.

The above-described laser beam can be various kinds of laser beams, suchas an excimer laser and CO₂ laser, in addition to a femtosecond laser.The above-described energy capable of transmitting through the medium isnot limited to the laser beam. Various kinds of active energy rays, suchas heat rays, infrared rays, ultraviolet rays, X rays, and radiation,for example, can be used.

When a femtosecond laser is not used and a laser beam having awavelength at which the reform medium layer 4 absorbs the laser beam isused, it is preferable that the reform medium layer 4 be made of amaterial whose light absorption wavelength is different from that of thesecond medium 2, as described above.

In the above preferred embodiments, energy for use in reforming ispreferably provided to the second medium 2 from thereabove, not in thefirst medium 1. However, in contrast to this, the energy may be providedfrom below the first medium 2 and guided into the first medium 1 toperform reforming.

In the above preferred embodiments, the first medium 1 is preferablymade of LiNbO₃, i.e., a piezoelectric material, so the first medium 1has the property of exciting a boundary acoustic wave usingpiezoelectric effect. Accordingly, it is preferable that reforming beperformed in the second medium 2, which is dielectric, not in the firstmedium 1.

As illustrated in the schematic front cross-sectional view of FIG. 8, areform medium layer 4A may preferably be made of a dielectric materialand arranged in contact with the IDT electrode 3. Here, the reformmedium layer 4A is disposed so as to cover the IDT electrode 3 andarranged to replace a portion of the second medium 2 at a side of theinterface X that is adjacent to the second medium 2. This structure isalso included in the structure in which the interface X is disposedbetween the first medium 1 and the second medium 2 and the reform mediumlayer 4A is disposed within the second medium 2.

As described above, the reform medium layer 4A may be in contact withthe interface X. In this case, in order to prevent a short circuit withthe IDT electrode 3, it is preferable to make the reform medium layer 4Aof an insulating material. The insulating material defining the reformmedium layer 4A is not particularly limited so long as it is reformed byprovided energy more readily than the second medium 2. Variousinsulating materials can be used. Examples thereof include a metallicoxide (e.g., zinc oxide (ZnO)), and a metallic nitride (e.g., SiN,aluminum nitride (AlN)).

As illustrated in FIG. 9, no reform medium layer may be disposed, andthe frequency adjustment may be achieved by reforming a portion of thesecond medium 2 by irradiation of energy emitted from outside the secondmedium 2. In this case, the reformed portion can be disposed adjacent tothe leading edge of the arrow B illustrated in FIG. 9 by focusing, forexample, a laser beam on an area adjacent to the interface X.Accordingly, in the present invention, a reform medium layer is notnecessarily required.

In the present invention, it is preferable that the reformed portion bearranged within a distance of λ from the interface X in the side inwhich the reformed portion is present. Accordingly, it is preferablethat the reform medium layers 4 and 4A be also be arranged within adistance of λ from the interface X. A boundary acoustic wave propagateswhile concentrating its energy on a portion within a distance of λ fromthe interface X in the direction in which the first medium 1 and thesecond medium 2 are laminated. Accordingly, frequency characteristicscan be effectively adjusted by the provision of the reformed portion andthe reform medium layer in a region within ±λ from the interface X in avertical direction.

In the above preferred embodiment, the reform medium layer 4 ispreferably made of gold. However, a metal other than gold or an alloymay be used. In other words, the reform medium layer 4 can be made ofany material that is diffused by provided energy and that achievesreforming. Examples of the above metal or alloy include, copper andsilver, for example, in addition to gold. The first medium 1 made ofLiNbO₃ may alternatively be made of another piezoelectric singlecrystal, such as, for example, LiNbO₃ having another cut angle andlithium tantalate (LiTaO₃). Alternatively, piezoelectric ceramics, suchas lead zirconate titanate (PZT) ceramics, may also be used.

The second medium 2 may also be made of various kinds of insulatingmaterial and dielectric material, such as silicon, glass, and siliconcarbide (SiC), for example, in addition to SiO₂. Alternatively, thesecond medium 2 may also be made of a piezoelectric material, such asZnO, Ta₂O₅, PZT, LiTaO₃, and LiNbO₃, for example.

Each of the first medium and the second medium may be a structure inwhich a plurality of material layers is laminated.

FIG. 10 illustrates dependence of transmittance of SiO₂ forming thesecond medium 2 on wavelength. FIG. 11 illustrates dependence ofabsorptance of various metals on wavelength.

When the second medium 2 is made of SiO₂, light having a wavelengthequal to or less than about 170 nm to about 180 nm is absorbed. Incontrast to this, in the case of gold, as illustrated in FIG. 11, lighthaving a wavelength equal to or greater than about 600 nm is reflectedby approximately 100%, whereas, in a wavelength range less than about600 nm, the absorptance is about 50%. Accordingly, when a laser beamhaving a wavelength greater than about 180 nm to transmit through thesecond medium 2 made of SiO₂ and having a wavelength less than 600 nm soas to be absorbed in the reform medium layer 4 is used, if the reformmedium layer is made of gold, reforming can be performed by diffusion ofgold, similar to the above preferred embodiment. One example of such alaser beam can be an excimer laser. As is clear from FIG. 11, the lightabsorption wavelength of copper and that of silver are different fromthat of the above-described SiO₂. Therefore, when the second medium ismade of SiO₂, copper and silver can also be used in forming the reformmedium layer within the second medium.

While preferred embodiments of the present invention have been describedabove, it is to be understood that variations and modifications will beapparent to those skilled in the art without departing the scope andspirit of the present invention. The scope of the present invention,therefore, is to be determined solely by the following claims.

1. A method for manufacturing a boundary acoustic wave device comprisingthe steps of: preparing the boundary acoustic wave device including afirst medium, a second medium laminated on the first medium, and aninterdigital transducer electrode arranged at an interface between thefirst medium and the second medium and being in contact with both of thefirst medium and the second medium, the boundary acoustic wave devicebeing arranged to utilize a boundary acoustic wave that propagatesthrough the interface; and reforming at least one of the first mediumand the second medium by externally providing the laminated structurewith energy that reaches an inside portion that is spaced away from anouter surface of the at least one of the first medium and the secondmedium so as to adjust a frequency of the boundary acoustic wave device.2. The method for manufacturing a boundary acoustic wave deviceaccording to claim 1, wherein, in the step of reforming the at least oneof the first medium and the second medium by externally providing the atleast one of the first medium and the second medium with the energy soas to adjust the frequency of the boundary acoustic wave device, theenergy is concentrated on a portion of the at least one of the firstmedium and the second medium, and the portion of the at least one of thefirst medium and the second medium is thus reformed.
 3. The method formanufacturing a boundary acoustic wave device according to claim 1,further comprising the step of forming a reform medium layer in at leastone of the first medium and the second medium, wherein in the step ofreforming the at least one of the first medium and the second medium soas to adjust the frequency, the reforming is performed by providing theenergy to the reform medium layer.
 4. The method for manufacturing aboundary acoustic wave device according to claim 3, wherein a lightabsorption wavelength of the reform medium layer is different from alight absorption wavelength of the at least one of the first medium andthe second medium, in which the reform medium layer is disposed, thereforming is performed by concentration of a laser beam defining theenergy on the reform medium layer, and the laser beam has a wavelengthat which the reform medium layer absorbs the laser beam.
 5. The methodfor manufacturing a boundary acoustic wave device according to claim 3,wherein each of the reform medium layer and the at least one of thefirst medium and the second medium is made of a material that isreformed by heat, and a temperature at which the reform medium layerreforms is lower than a temperature at which the at least one of thefirst medium and the second medium, in which the reform medium layer isdisposed, reforms.
 6. The method for manufacturing a boundary acousticwave device according to claim 1, wherein, when a wavelength of the IDTelectrode is λ, in reforming the at least one of the first medium andthe second medium so as to adjust the frequency, the reforming isperformed by providing the energy to a region within a distance of λfrom the interface in a direction in which the first medium and thesecond medium are laminated.
 7. The method for manufacturing a boundaryacoustic wave device according to claim 1, wherein a femtosecond laserbeam is used to supply the energy.